1. Field of the Invention
The invention relates to a titanium-based material that is formed so that a highly conductive film is formed on a surface of a base material made of titanium or titanium alloy. Specifically, the invention relates to a titanium-based material that is able to ensure adhesion of the film, a method of manufacturing the titanium-based material, and a fuel cell separator made of a titanium-based material.
2. Description of the Related Art
A polymer electrolyte fuel cell that uses an electrolyte membrane is operable at low temperatures, and may be reduced in size and weight. Therefore, application of the polymer electrolyte fuel cell to a mobile unit, such as an automobile, has been studied. Particularly, a fuel-cell vehicle equipped with a polymer electrolyte fuel cell attracts social interest as an ecology car.
As shown in FIG. 10, such a polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) 95 as a major component, and the MEA 95 is held between separators 96 each having a fuel (hydrogen) gas flow passage and an air gas flow passage. By so doing, one fuel cell 90 called a single cell is formed. The membrane electrode assembly 95 has a structure such that an anode electrode (anode catalyst layer) 93a is stacked on one side of an electrolyte membrane 91, which is an ion exchange membrane, and a cathode electrode (cathode catalyst layer) 93b is stacked on the other side of the electrolyte membrane 91. The anode catalyst layer 93a includes a gas diffusion layer 94a. The cathode catalyst layer 93b includes a gas diffusion layer 94b. 
A titanium-based material may be used as the material of the separator of a fuel cell. The above material has a passivation oxide film on its surface layer. The oxide film is anticorrosive under a general environment, so it is desirable to use the oxide film as the material of the separator. The separator of a fuel cell is energized during power generation of the fuel cell. However, because of the oxide film, there is a possibility that contact resistance increases and, therefore, conductivity to the separator is impaired. This results in a decrease in the power generation characteristic of the fuel cell. Then, a carbon film is formed (deposited) on the surface of a titanium base material (base material made of titanium) to ensure the conductivity of the separator.
Incidentally, the thickness of such a titanium base material is adjusted by rolling. During the rolling, a TiCx layer (rolling-altered layer) is formed on the surface of the titanium base material because of rolling oil. The rolling-altered layer easily corrodes, so it may deteriorate the anticorrosion property of the titanium base material. Then, when a titanium base material is rolled, generally, a rolling-altered layer is removed from the titanium base material, and a carbon film is deposited on the surface from which the rolling-altered layer has been removed.
However, when a carbon film is deposited on the surface of the titanium base material, the adhesion of the carbon film is not so good, so there is a possibility of peeling, or the like, of the carbon film. In view of the above, there is proposed, for example, a titanium-based material that is formed so that a transition metal element, such as Ti and Zr, or a metalloid element, such as Si and B, is formed as an intermediate layer between an oxide film of a base material and a carbon film (see, for example, Japanese Patent Application Publication No. 2004-185998 (JP-A-2004-185998)). By forming the above intermediate layer, it is possible to improve the adhesion between the titanium base material and the carbon film.
However, when a titanium-based material is manufactured as in the case of JP-A-2004-185998, it is necessary to deposit the intermediate layer on the surface of the titanium base material before the carbon film is formed. This increases a manufacturing process and may lead an increase in manufacturing cost. In addition, depending on selection of the material that constitutes the intermediate layer, the adhesion may not be sufficiently ensured, so it is not reliable enough.